Common mode (CM) inductors are used to reduce common mode electromagnetic interference (EMI) noise by imposing impedance that exists primarily in the common mode (i.e., each line to ground). Such inductors saturate magnetically (i.e., a high flux density exists in the core) from current imbalances. Current imbalances can come from external noise and imbalance, but also from the inductors' own leakage inductance, which occurs between windings.
Common mode transformers can be used in three-phase motor drive systems to inject a signal into power windings to help cancel out CM noise. To be effective, coupling between the drive winding and the power windings must be as complete as possible to allow for complete magnetic cancellation. In some systems, a fourth drive winding is wound in the same sense as the total power winding combination.